The present invention relates to a torque sensor, and more particularly to a torque sensor for an electric power steering apparatus for alleviating the steering torque of a driver by causing the power of a motor to be directly applied to a steering system.
An electric power steering apparatus has a motor in a steering system, and alleviates the steering torque of the driver by controlling the power supplied from the motor by using a controller. A conventional electric power steering apparatus has a steering torque detecting unit (torque sensor) provided in a steering gearbox for detecting a steering torque applied to a steering shaft coupled to a steering wheel. Its detected value is inputted to the controller, and is supplied as a reference signal for allowing the motor to generate an appropriate assist steering torque.
As the above-described steering torque detecting unit, a magnetostriction type is known in addition to a torsion bar type which makes use of the torsion of a torsion bar provided between input and output shafts of a pinion. As one example of the magnetostriction-type torque detecting unit, magnetostrictive films of such as Ni—Fe platings are provided with predetermined axial widths on the surface of the steering shaft coupled to the steering wheel, in such a manner as to assume magnetic anisotropies of mutually opposite directions at two upper and lower portions. When the steering torque is applied to the magnetostrictive films, an inverse magnetostriction characteristic occurring on the basis of the magnetic anisotropies is detected by making use of ac resistances or the like of coils disposed around the magnetostrictive films. Such torque sensors are disclosed in patent document 1 and patent document 2.
FIG. 6 is a schematic diagram of the relationship of layout among the exciting coil, the detection coils, and the magnetostrictive films in a torque sensor 100 such as the one described above. A magnetostrictive film 102 is formed on the surface of a steering shaft 101, and a magnetostrictive film 103 is formed thereon at an interval with the magnetostrictive film 102. An exciting coil 104 is disposed in the vicinity of these magnetostrictive films with a very small gap provided between the exciting coil 104 and the magnetostrictive films. An exciting-voltage supply source 105 is connected to the exciting coil 104. Further, a detection coil 106 is disposed in the vicinity of the magnetostrictive film 102 with a very small gap provided therebetween, while a detection coil 107 is disposed in the vicinity of the magnetostrictive film 103 with a very small gap provided therebetween.
In the torque sensor 100 shown in FIG. 6, when torque is applied to the steering shaft 101, the torque is also applied to the magnetostrictive films 102 and 103. The inverse magnetostrictive effect occurs in the magnetostrictive films 102 and 103 in correspondence with this torque. For this reason, when a high-frequency ac voltage (exciting voltage) is supplied from the exciting-voltage supply source 105 to the exciting coil 104, a change in the magnetic field due to the inverse magnetostrictive effect of the magnetostrictive films 102 and 103 based on the torque can be detected by the detection coils 106 and 107 as a change in impedance or induced voltage. From this change in impedance or induced voltage, it is possible to detect the torque applied to the steering shaft 101.
One example of such an inverse magnetostrictive characteristic is shown in FIG. 7. In FIG. 7, the abscissa shows the steering input torque, while the ordinate shows the impedance or induced voltage detected by the detection coils when an ac voltage is applied to the exciting coil. A curve C10 shows the change in impedance or induced voltage detected by the detection coil 106 when an external magnetic field is absent, while a curve C11 shows the change in impedance or induced voltage detected by the detection coil 107 when an external magnetic field is absent. In the detection by the detection coil 106, the impedance or induced voltage increases as the steering torque changes from negative to positive, and the impedance or induced voltage assumes a peak value P1 when the steering torque has assumed a positive value T1, while it decreases when the steering torque is more than T1. On the other hand, in the detection by the detection coil 107, the impedance or induced voltage increases as the steering torque changes from positive to negative, and the impedance or induced voltage assumes the peak value P1 when the steering torque has assumed a negative value −T1, while it decreases when the steering torque is increased. As shown in FIG. 7, the steering torque-impedance (induced voltage) characteristic obtained by the detection coil 106 and the steering torque-impedance. (induced voltage) characteristic obtained by the detection coil 107 show substantially convex shapes. The steering torque-impedance (induced voltage) characteristic obtained by the detection coil 106 and the steering torque-impedance. (induced voltage) characteristic obtained by the detection coil 107 become substantially symmetrical about the axis of ordinates by reflecting the magnetic anisotropies which assume mutually opposite directions at the two upper and lower magnetostrictive films mentioned earlier. In addition, a straight line L10 shows a value in which the characteristic curve C11 detected by the detection coil 107 is subtracted from the characteristic curve C10 detected by the detection coil 106. The straight line L10 shows that its value becomes zero when the steering torque is zero, and that its value changes substantially linearly with respect to the change in the steering torque in the range R of the steering torque. The magnetostriction-type torque detecting unit outputs a detection signal corresponding to the direction and magnitude of the input torque by using a region which is considered to exhibit a substantially fixed gradient particularly in the vicinity of a torque neutral point among such characteristic curves C10 and C11. In addition, by using the characteristic of the straight line L10, it is possible to detect the steering torque from the values of the detection coils 106 and 107.
Next, a description will be given of a case where an external magnetic field is present in such a torque sensor. When the external magnetic field is present, the magnetic properties of the system including the magnetostrictive films 102 and 103 and the steering system formed of a magnetic material undergo a change. As a result, as shown in FIG. 7, the characteristic curve obtained by the detection coil 106 and the characteristic curve obtained by the detection coil 107 shift as shown by a dotted line C20 and a dotted line C21, respectively. Consequently, a straight line obtained by the difference between the characteristic curves C21 and C22 also shifts as shown by a dotted line L20, and assumes a value which is not zero even when the steering torque is zero, resulting in the occurrence of a so-called midpoint offset. In the case of the electric power steering apparatus, the midpoint offset of the output of the torque sensor constitutes a left-right difference in the steering assist force, and imparts an uneasy feeling to the driver who performs the steering. Therefore, it is necessary to perform zero point correction, i.e., neutral point adjustment, in order to obtain the steering torque from the detected values of the detection coils 106 and 107. For this reason, the neutral point adjustment is normally essential after the installation of the torque sensor on the electric power steering apparatus.
However, with the conventional steering torque detecting unit, since no special magnetic shield section is provided, if a change in the external environment before and after the installation of the torque detecting unit in the vehicle is taken into consideration, there is a problem in that it is, difficult to perform the neutral point adjustment of the torque detection signal by the torque detecting unit as a single unit. Namely, if the magnetic field of the vicinity of the torque detecting unit changes due to the approach of a magnet or the like from the outside after the neutral point adjustment of the torque detection signal is effected as a single unit and the torque detecting unit is installed in the vehicle, the neutral point of the torque detection signal changes. For example, since the characteristic of the steering torque when steering without driving is effected differs according to the rotating direction of the steering wheel, the neutral point of the torque detection signal must be readjusted. Accordingly, patent document 3 proposes a structure in which a magnetic shield is provided in the vicinity of the magnetostrictive films of the torque sensor.
[Patent Document 1]
JP-A-2001-133337
[Patent Document 2]
JP-A-2002-168706
[Patent Document 3]
JP-A-2001-296193
However, in the magnetic shield section disclosed in patent document 3, although a magnetic shield is provided for only the surroundings of the magnetostrictive films by a casing of a nonmagnetic highly electrically conductive material, the effect of an external magnetic field is exerted on not only the magnetostrictive films but the steering shaft formed of a magnetic material, so that the steering shaft is magnetized by the external magnetic field. Because the steering shaft is magnetized, there arises the problem that the characteristic of the torque sensor changes. For this reason, if the magnetic shield is provided only for the surroundings of the magnetostrictive films, there is a problem in that it is difficult to suppress the effect of the external magnetic field on the torque sensor.